1. Field of the Invention
This invention relates to an illumination system, and more particularly to an illumination system for EUV lithography with wavelengths less than 193 nm.
2. Description of the Prior Art
To reduce the structural widths for electronic components, especially in the submicron range, it is necessary to reduce the wavelength of the light used for microlithography. For example, lithography with soft x-rays is conceivable with wavelengths smaller than 193 nm. U.S. Pat. No. 5,339,346 disclosed an arrangement for exposing a wafer with such radiation. An illumination system for soft x-rays, so-called EUV radiation, is shown in U.S. Pat. No. 5,737,137, in which illumination of a mask or a reticle to be exposed is produced using three spherical mirrors.
Field mirrors that show good uniformity of output of an exposure beam at a wafer in a lithographic system have been disclosed in U.S. Pat. No. 5,142,561. The exposure systems described therein concern the contact exposure of a wafer through a mask with high-energy x-rays of 800 to 1800 eV.
EUV illumination systems for EUV sources have been disclosed in EP 99 106 348.8 (U.S. application Ser. No. 09/305017) and PCT/EP99/02999. These illumination systems are adapted to synchrotron, wiggler, undulator, Pinch-Plasma or Laser-Produced-Plasma sources.
Scanning uniformity is a problem of the aforementioned scanning exposure systems in illuminating a slit, particularly a curved slit. For example, the scanning energy obtained as a line integral over the intensity distribution along the scan path in a reticle or wafer plane may increase toward the field edge despite homogeneous illumination intensity because of the longer scan path at the field edge for a curved slit. However, scanning energy and with it scanning uniformity may also be affected by other influences, for example coating or vignetting effects are possible. The curved slit is typically represented by a segment of a ring field, which is also called an arc shaped field. The arc shaped field can be described by the width delta r, a mean Radius R0 and the angular range 2xc2x7xcex10. For example, the rise of the scanning energy for a typical arc shaped field with a mean radius of R=100 mm and an angular range of 2xc2x7xcex10=60xc2x0 is 15%.
It is an object of the present invention to provide an illumination system for a projection exposure system in which the scanning energy is uniform, or can be controlled to fit a predetermined curve.
This and other objectives of the present invention are achieved by shaping a field lens group in an illumination system of a generic type so that the illuminated field is distorted in an image plane of the illumination system perpendicular to the scanning direction. In this plane the mask or reticle of a projection exposure system is located.
The term xe2x80x9cfield lens groupxe2x80x9d is taken to describe both field mirror(s) and field lens(es). For wavelengths xcex greater than 100 nm the field lens group typically comprises refractive field lens(es), but mirrors are also possible. For wavelengths in the EUV region (10 nm less than xcex less than 20 nm) the field lens group comprises reflective field mirror(s). EUV lithography uses wavelengths between 10 nm and 20 nm, typically 13 nm.
According to the present invention it is possible to determine the necessary distortion to obtain a predetermined intensity distribution. It is advantageous for a scanning system to have the capability of modifying the intensity distribution perpendicular to the scanning direction to get a uniform distribution of scanning energy in the wafer plane. The scanning energy can be influenced by varying the length of the scanning path or by modifying the distribution of the illumination intensity. The present invention relates to the correction of the distribution of the illumination intensity. In comparison to stepper systems where a two-dimensional intensity distribution has to be corrected, a scanner system only requires a correction of the distribution of the scanning energy.
In one embodiment of the present invention, the illumination intensity decreases from the center of the field to the field edges by means of increasing distortion. The intensity is maximum at the field center (xcex1=0xc2x0) and minimum at the field edges (xcex1=xc2x1xcex10). A decrease of the illumination intensity towards the field edge permits a compensation for an increase of the scan path so that the scanning energy remains homogeneous.
The present invention also provides for the illumination intensity to increase from the center of the field to the field edges by means of decreasing distortion. This correction can be necessary if other influences like layer or vignetting effects lead to a decreasing scanning energy towards the field edges.
Preferably, the field lens group is designed so that uniformity of scanning energy in the range of xc2x17%, preferably xc2x15%, and very preferably xc2x13%, is achieved in the image plane of the illumination system.
The field lens group is shaped so, that the aperture stop plane of the illumination system is imaged into a given exit pupil of the illumination system. In addition to the intensity correction, the field lens group achieves the correct pupil imaging. The exit pupil of the illumination system is typically given by the entrance pupil of the projection objective. For projection objectives, which do not have a homocentric entrance pupil, the location of the entrance pupil is field dependent. In such a case, the location of the exit pupil of the illumination system is also field dependent.
The shape of the illuminated field according to this invention is rectangular or a segment of a ring field. The field lens group is preferably shaped such that a predetermined shaping of the illuminated field is achieved. If the illuminated field is bounded by a segment of a ring field, the design of the field lens group determines the mean radius R0 of the ring field.
It is advantageous to use a field lens group having an anamorphotic power. This can be realized with toroidal mirrors or lenses so that the imaging of the x- and y-direction can be influenced separately.
In EUV systems the reflection losses for normal incidence mirrors are much higher than for grazing incidence mirrors. Accordingly, the field mirror(s) is (are) preferably grazing incidence mirror(s).
In another embodiment of the present invention the illumination system includes optical components to transform the light source into secondary light sources. One such optical component can be a mirror that is divided into several single mirror elements. Each mirror element produces one secondary light source. The mirror element can be provided with a plane, spherical, cylindrical, toroidal or an aspheric surface. Theses single mirror elements are called field facets. They are imaged in an image plane of the illumination system where the images of the field facets are at least partly superimposed.
For extended light sources or other purposes it can be advantageous to add a second mirror that is divided in several single mirror elements. Each mirror element is located at a secondary light source. These mirror elements are called pupil facets. The pupil facets typically have a positive optical power and image the corresponding field facets into the image plane.
The imaging of the field facets into the image plane can be divided into a radial image formation and an azimuthal, image formation. The y-direction of a field facet is imaged in the radial direction, and the x-direction is imaged in the azimuthal direction of an arc shaped field. To influence the illumination intensity perpendicular to the scanning direction the azimuthal image formation will be distorted.
The imaging of the field facets is influenced by the field lens group. It is therefore advantageous to vary the azimuthal distortion by changing the surface parameters of the components of the field lens group.
The field lens group is shaped such that the secondary light sources produced by the field facets are imaged into a given exit pupil of the illumination system.
With a static design of the field lens group, a given distribution of the illumination intensity, the shaping of the illuminated field and the pupil imaging can be realized. The effects that are known can be taken into account during the design of the field lens group. But there are also effects that cannot be predicted. For example, the coatings can differ slightly from system to system. There are also time dependent effects or variations of the illumination intensity due to different coherence factors, so called setting dependent effects. Therefore, actuators on the field mirror(s) are preferably provided in order to control the reflective surface(s).
The distortion, and thus the illumination intensity, can be modified using the actuators. Since the surface changes also affect the pupil imaging, intensity correction and pupil imaging are regarded simultaneously. The surface changes are limited by the fact that the directions of centroid rays that intersect the image plane are changed less than 5 mrad, preferably less than 2 mrad, and very preferably less than 1 mrad.
It is advantageous to reduce the number of surface parameters to be controlled. To influence the illumination intensity, and thus the scanning intensity, only the surface parameters that influence the shape of the mirror surface(s) perpendicular to the scanning direction will be modified. These are the x-parameters if the scanning direction is the y-direction.
A particularly simple arrangement is obtained when the actuators for controlling the field mirror surface are placed parallel to the scan direction or the y-axis of the field mirror, for example in the form of a line or beam actuator.
The present invention also provides for a projection exposure system for microlithography using the previously described illumination system. A mask or reticle is arranged in the image plane of the illumination system, which is also an interface plane between the illumination system and projection system. The mask will be imaged into a wafer plane using a projection objective.
The illumination of the wafer is typically telecentric. This means that the angles of the chief rays regarding the wafer plane are smaller than xc2x15 mrad. The angle distribution of the chief rays in the reticle plane is given by the lens design of the projection objective. The directions of the centroid rays of the illumination system must be well adapted to the directions of the chief rays of the projection system in order to obtain a continuous ray propagation. The telecentricity requirement is fulfilled in this invention when the angular difference between the centroid rays and the chief rays does not exceed a given degree in the plane of the reticle, for example xc2x110.0 mrad, preferably xc2x14.0 mrad, and very preferably 1.0 mrad.
For scanning lithography it is very important that the scanning energy in the wafer plane is uniform. With the previously described illumination system it is possible to achieve uniformity values of scanning energy in the wafer plane in the range of xc2x17%, preferably xc2x15%, and very preferably xc2x13%.
The present invention also provides for a method for calculating the magnification xcex2s for the azimuthal imaging of the field facets for a predetermined distribution of scanning energy. With the knowledge of the azimuthal magnification xcex2s the design of the field lens group can be determined.
If the predicted distribution of scanning energy in the wafer plane is not obtained, the scanning energy can be corrected using the actuators of the field mirror(s). From the difference between the predicted and measured distribution of scanning energy the magnification for the azimuthal imaging of the field facets, and thus the necessary surface corrections, can be calculated.